In color reproduction, the color accuracy of graphic art color separation negatives are generally verified using color proofing systems prior to making the corresponding printing plates. The color proofing systems must provide a consistent representation of the final color print. It is well known in the art that the halftone dot retention plays a significant role in the color rendition of the final image. Slight changes in the dot reproduction curve can cause significant changes in visual color perception.
Several different types of color proofing systems are well known in the literature. Both digital and conventional proofing systems are available commercially. In digital systems, digitized data is used to directly image the color proofing materials. However, conventional proofing systems are primarily used when verifying the color accuracy of graphic art color separation films. Conventional color proofing systems consist of two types: overlay and surprint. Overlay systems provide a convenient and quick way to visualize a representative color on a transparent substrate. A typical overlay film construction includes a polyester substrate and photosensitive color layer. Optional protective layers may also be included over the surface of the photosensitive color layer. A color proof is constructed by individually imaging and developing each representative color proofing film. The color imaged films are then laid upon each other in register on a paper stock, thus providing a quick and simple color proof composite. Examples of these types of constructions may be generally found in U.S. Pat. Nos. 3,136,637; 4,304,836; 4,482,625; 4,634,652; 4,748,101; 4,952,482; and 5,258,261.
A surprint proof is generated by successively superimposing different colored layers upon each other on a single sheet. The surprint color proofing materials are composed of two primary types: photosensitive precolored sheets and photosensitive colorless sheets whose latent image may be toned with pigments, inks or dyes. Some examples of post-colored proofing materials may be found in U.S. Pat. Nos. 3,060,024; 3,634,087; and DE 3,336,431.
An example of one type of precolored proofing system includes photosensitive constructions that are imaged prior to lamination to the receptor or intermediate sheet. Representative examples of these types of systems are described in U.S. Pat. Nos. 4,482,625 and 4,304,836.
Another type of precolored proofing system includes photosensitive constructions that are imaged and developed after lamination to a receptor or intermediate sheet. For example, U.S. Pat. No. 3,671,236 describes a negative acting presensitized color proofing element comprising a carrier sheet having a smooth release surface, a diazo sensitized color layer and a non-photoactive barrier layer upon which may be coated a pressure sensitive adhesive or a thermal adhesive as described in U.S. Pat. No. 4,656,114. The developing media for this system is a 50:50 mixture of n-propanol and water. The barrier layer is present as a carrier for the image and to reduce interaction between the adhesive and the photoactive layer. U.S. Pat. Nos. 4,650,738; 4,751,166; 4,808,508; 4,929,532; and 5,008,174; EP 365,356 and EP 365,357 patent applications describe improvements on this construction by eliminating the need for a barrier layer between the diazo based photoactive layer and the adhesive. The developer used in these systems are aqueous alkali solutions containing greater than 3% surfactant.
U.S. Pat. No. 4,596,757 discloses a further improvement on the construction described in U.S. Pat. No. 3,671,236 by utilizing a photo monomer system in place of the diazonium binders. Again an aqueous alkali developer is used which contains high concentrations of a surfactant (the example cited used 8.5%).
U.S. Pat. No. 5,248,583 describes a construction which utilizes a photoactive color layer containing a photo-oligomer as the photopolymerizable component and a photopolymerizable barrier layer to provide a proofing element that can be developed in a low solids aqueous developer having a pH of 10.2. This construction allows one to develop the image using a more environmentally compatible effluent since the aqueous developer contains less caustic and less ingredients than those used in the previously described systems.
To achieve a system that is developable in a developing solution having a more neutral pH, the binders used in the photosensitive proofing formulation typically make the image more susceptible to attack by the developer. When the resistively developable image is swelled or attacked by the developer it becomes increasingly more difficult to hold the highlight dots (0.5-10% halftone dots) without sacrificing the shadow areas (90-99.5% halftone dots). The shadow areas are sacrificed when the photosensitive layer is overexposed to achieve sufficient cross-link density in the highlight dots to resist developer attack. However, when a photosensitive layer is overexposed, the shadow areas begin to fill-in due to halation effects. This over-exposure also increases the overall dot gain, which significantly affects the color rendition. Therefore, there exists a need for a proofing system that maintains highlight halftone dots without over-exposure and is developable in an aqueous developer having low solids and a pH less than 10.5.
Some attempts have been made to provide proofing elements that are coated out of water and developed with water. For example, U.S. Pat. No. 5,443,937 describes a photosensitive element based a photosensitive polymeric diazo resin. However, polymeric diazo resins typically generate color upon exposure to light which effects the final color of the image. In the printing industry, the color rendition of the proof is a critical factor of the proofing system. Any changes must be compensated for in the initial formulation of the color coatings. Adjustments to the formulations are a tedious task since the composition has to be coated, dried and exposed before an evaluation of the color can be performed. In addition, negative images using diazonium resins must be post exposed to light to stabilize the color. This post exposure step adds an additional step to the proof making process. Therefore, there is still a need for a water coatable and water developable color proofing element that does not have the disadvantages of color shifts due to the generation of color by the diazo resins and does not require additional process steps.
U.S. Pat. No. 4,041,204 describes a photosensitive sheet using a water coatable and aqueous developable photosensitive color layer. Even though the examples disclose the use of water to develop the image, the sheet is soaked in water for about a minute before spraying to complete the removal of the unexposed areas. In other examples, the image was developed with a 4% solution of hydrogen peroxide. Resolution is not discussed and would not be a critical issue in stencil applications.